Dedicated Controls
2715 User Manual
4–15
When you press [VID FLTR], the 2715 automatically selects a narrower video
filter bandwidth approximately 1/100 of the resolution BW (specifying particular
filter bandwidth using the UTIL menu is described later in this section). The
narrow video filter dramatically reduces the noise and enhances the visibility of
narrow-band signals. Care must be taken, though, because it will also reduce the
indicated amplitudes of wideband signals such as video modulation and short
duration pulses.
The following steps demonstrate the effects of the video filter:
1. Ensure that the 2715 is set as in the settings box on page 4–14, and note if
the signal peaks at 700 MHz, 800 MHz, and 900 MHz are visible.
2. Press [VID FLTR].
The red LED below the key lights, indicating that a narrow video filter is
being used, and the bandwidth of the filter is indicated on screen.
Notice how much less noisy the lower portion of the spectral display appears. By
filtering the noise, it is sometimes possible to reveal low-level signals that are in
the noise. This is the primary reason for using a video filter. The signal peaks at
700 MHz, 800 MHz, and 900 MHz (the seventh, eighth, and ninth harmonics of
the calibration signal) should now be visible. There are small differences from
instrument to instrument, but you should be able to spot the peaks above the
noise.
Video filtering works well for continuous wave and other narrow-band signals,
but when examining pulsed or wideband signals such as television video
(especially the sync pulses), a video filter may prevent you from accurately
seeing signal characteristics in much the same way that using a resolution
bandwidth that is too narrow does.
Although it may not have been apparent, the sweep speed also decreased in order
to accommodate the longer time constant of the video filter. Just as with the
resolution bandwidth filter, a signal needs more time to reach its peak amplitude
when propagating through a narrow video filter.
3. Press [VID FLTR] again to toggle the video filter off.
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